How can I copy or duplicate several objects in one step?
I was about to answer "Array modifier", but your case may be more complex. My way to accomplish this:


I make a separate scene just for one component (e.g. a window frame to be duplicated, or in your case, one full story), and model it there. 
After I'm done, I group it as one component.
Lastly, in the main scene, I instanced the group (Shift-A -> Group Instance) as many as I
needed. Each instances are free to be transformed.


Here's an example from a building I modeled, the original repeatable objects are collected in a scene for easy measurement control:



What I like about this approach, is that we can always modify the base component in its own scene, and all instances in the main scene will be updated. And unlike linked-duplicates, we can add as many object to the group as needed.